Tumor progression and metastasis are in part driven by proliferating microvessels that provide oxygen and nutrients to the growing cancer cells. Angiogenic vessels exert an instructive role as a vascular niche by providing paracrine and angiocrine modes of regulation which include secretion and processing of various growth factors and extracellular matrix constituents that in turn influence tumor and endothelial cells in a bidirectional way. Perlecan, a major heparan sulfate proteoglycan secreted by endothelial cells, is a complex regulator of vascular biology and tumor angiogenesis. Perlecan shows a clear functional dichotomy. The parent proteoglycan is pro-angiogenic by acting as a co-receptor for FGF2 and VEGFA via the N-terminal heparin sulfate chains. In contrast, the C-terminal portion of perlecan, named endorepellin, is anti-angiogenic. We have recently discovered that endorepellin binds with high affinity to both 21 integrin and VEGFR2. This dual receptor binding induces co-internalization and physical downregulation of both receptors, activation of SHP-1 and attenuation of VEGFA transcription. These findings provide a new paradigm for the activity of an anti-angiogenic protein and mechanistically explain the specificity of endorepellin for endothelial cells, the only cells that simultaneously express both 21 integrin and VEGFR2. Our long-term objectives are to expand these observations by investigating the binding mode of endorepellin and its various LG modules to the ectodomain of VEGFR2, and decipher the downstream signaling pathway evoked by this natural antagonist of two major angiogenic receptors. Specifically we plan to: [1] Investigate the precise mode through which endorepellin and its LG modules bind to the ectodomain of VEGFR2. [2] Decipher the downstream signaling pathways evoked by endorepellin on vascular endothelial cells, and [3] Generate novel mouse models of tumorigenesis where endorepellin and perlecan are conditionally overexpressed or ablated, respectively, in the endothelia of growing tumors. The expected results should generate a coherent and unified mode of action for endorepellin, identify new target proteins involved in its signaling pathway which could be potential novel targets for anti-angiogenesis therapy, and establish its in vivo role as an anti-angiogenic factor using unambiguous gain- and loss-of-function experiments through endothelial cell-specific gene targeting.